Mark the required dimensions on the surface of the sheet using marking tool and straightedge.
Cutting copper sheet with a torch.
While you can still cut the copper material with the plasma cutter you should not force it and cut too fast.
Measure the dimensions needed with tape measure.
Take your time and let the cutter work its way through the copper for a straighter and even cut.
Once the copper sheet has been marked where it needs to be cut the sheers blades are positioned on each side of the sheet as would be regular scissors when cutting paper.
Work gloves must be worn during the cutting process as the cutting edge of the copper is very sharp.
Begin cut with metal cutting sheer.
The sheers handles are brought together to cut through the sheet.
Cutting slower will also give you plenty of opportunity to blow away the bits and pieces of copper and dust caused by the cutting process.
For higher gauges meaning thinner copper you can easily do it with standard hardware store metal snips.
They look like big scissors and work the same way.